Flock Raiser feed?

What is the difference between Purina Flock Raiser and feed specific to layers? I read something on here about switching chicks from chick feed to layer too early, which caused kidney failure, I think, due to too much calcium. Someone mentioned that this Flock Raiser was good for any age chicken.
I always followed the directions on the bag and switched the young ones off chick starter at about eight weeks. All the young ones are on layer now, and the only hen from the older batch (from June) is laying perfectly. I haven't seen any health issues at all that would be related to kidneys, as far as I know (two roosters have a slight respiratory infection).
I'm just curious to clear this up in case we do get any more chicks.
Thanks!

The Purina Flock Raiser doesn't have added calcium so can be fed to a mixed flock (different ages). I feed it and have a container of oyster shell for them to take as needed. My understanding is that the calcium in layer feed can be harmful to young chicks. I did have a broody last August with 7 chicks which I separated from the rest (sort of a coop within a coop). The babies and the mother ate chick starter. I think they were about 6 weeks when I gradually integrated them back in with the flock, and then they all ate the Flock Raiser. The chicks are now grown and laying eggs. They all seem to be doing very well on the Flock Raiser. Healthy looking birds.